From a linguistic perspective, the determining thing is the constituent of sentence which is the focus of the question. If it's the subject, object, or verb you get ydy -- draig ydy hi?, hi ydy'r draig?, ydy hi'n mynd?. If it's anything else -- an adverbial phrase or prepositional phrase -- you put the question form of this phrase first -- ble, pryd, am faint o'r gloch, sut, etc. -- followed by (y) mae.
cinio is a true noun meaning 'lunch' or dinner'. So 'having lunch/dinner' is usually cael cinio
If ever you are unsure, just look the word up on, say, www.gweiadur.com (you need to register, but it is free) - that will list verb and noun meanings in separate paragraphs if the word does happen to be used as both (dechrau and newid, for example).
Similarly, if you use the 'Ap Geiriaduron' smart-phone app (it's free), that will list verb meanings with the abbreviation 'be' (for berfenw, verb-noun) and noun meanings as eg or eb (respectively enw gwrywaidd, 'masculine noun', and enw benywaidd, 'feminine noun')
(A 'verb-noun' is the basic form of a verb used to list it in a dictionary, and what we use after dw i'n ... etc.)
Welsh does not have an infinitive in that sense. A verb-noun (berfenw) is the basic, uninflected form of a verb in Welsh. It is similar to a Latin gerund and the xxx-ing form of verbs in English. For convenience it is often translated as the equivalent of the English infinitive form 'to xxx' or as 'xxx-ing'
The short answer is 'yes', but the long answer is that it is far more complicated for a number of reasons.
Firstly, linguists tend to describe different languages in different ways even when there are no real differences. The first grammars were written in Latin about Greek, and verbs were listed by 1st person singular, present indicative active. This was fine for Greek as it distinguished the two main conjugations but it was useless when transferred to Latin as it failed to distinguish the main conjugations. Derived languages, such as French, list by infinitive. Old Irish was listed by 3rd person singular present indicative active. Modern Irish and Gaelic are listed by verb stem, which, as in English, is identical to the singular imperative. You then have to add various different endings to make the verb-noun. Welsh starts with the verb-noun and then you have to take off various endings to make the verb stem. You will see that this means the basic structure of Welsh and Gaelic is identical, but you just list different parts in the dictionary. The Welsh system is unique so far as I know, but it has the great advantage that it is much easier to take the various endings off than to add them on as you don't know which one to add. In Gaelic we always struggle with which one for which verb, a problem that has simply been eliminated in the Welsh system.
The one -ing word in Modern English is in fact replacing two words in Middle English: the -ing word which was a noun (he heard the ticking of the clock) and the -and word which was an adjective (the tickand clock was annoying him). The noun also replaces some uses of the infinitive in Norman (and Modern) French. But it is also used in the the clock is ticking construction not found in French or German. This distinctive English structure is clearly borrowed from our Celtic neighbours.
So yes the infinitive is sometimes the best translation, probably because of the Norman French influence on English
Could you give an example of what you mean by "Welsh starts with the verb-noun and then you have to take off various endings to make the verb stem"? I'm not familiar enough with Welsh to recognise what is being referred to here.
As a secondary point: doesn't Spanish also use the construction "The clock is ticking"? (Duo gives the translation "Estoy corriendo" for "I am running".) If so, was Spanish also influenced by a Celtic language?
Yes it will be good to clarify with an example. Suppose you want to say 'the fire burnt' or 'the fire was burning' in Welsh or Gaelic. Let us look at the translations first, then what it says in the dictionary:
'the fire burnt' W: llosgodd y tân, G: loisg an teine
'the fire was burning' W: roedd y tân yn llosgi, G: bha an teine a' losgadh
Now clearly the verb is much the same and the sentence structures are identical, but let's see what we find in the dictionary:
In a Gaelic dictionary, we find the word loisg ('burn') and it tells you to add an ending to get losgadh ('burning').
But in a Welsh dictionary, it tells you the word is llosgi ('burning') and you have to take off the ending to get the stem llosg ('burn') from which you get llosgodd.
So there isn't too much difference between the languages but what is different is what they tell you in the dictionary. In the Welsh dictionary, they give you the word for 'burning', whereas in the Gaelic dictionary they give you the stem, i.e. 'burn', and then tell you how to modify it to make the verb-noun.
Does that help?
As for the Spanish, that is very interesting. That structure does not come from Latin. Linguist tend to gloss over what doesn't seem to come from a 'posh' language, but I would assume it comes from Celtic. There were lots of Celts in Spain.